Nylon 6,6 is produced from two chemical ingredients, hexamethylenediamine (HMD) and adipic acid (AA). The HMD is, in turn, produced by the hydrogenation of adiponitrile (ADN). ADN is a linear alpha, omega dinitrile containing 6 carbon atoms. In the conversion of ADN to HMD, a major intermediate formed is aminocapronitrile (ACN), which has a nitrile group at one end of the molecule and an amine group at the other. Another intermediate that is formed is tetrahydroazipine (THA), which is formed by the addition of one molecule of hydrogen to ACN, followed by cyclization and elimination of ammonia. THA also exists as various oligomers with other molecules present in the system, such as HMD and ACN, and this mixture of THA and its oligomers is collectively referred to as "polarographically reducible impurities" (PRI), since analysis for the mixture is done by electrochemical reduction using a dropping mercury electrode. The presence of PRI in the HMD is undesirable, because it causes the Nylon 6,6 polymer to be of inferior quality. When HMD is produced by completely or nearly completely hydrogenating ADN, most of the THA is hydrogenated to hexamethyleneimine (HMI), which is easily removed from the HMD by distillation. The amount of PRI that exists in the crude HMD immediately following hydrogenation is less than 50 ppm, and can be controlled to acceptable levels in the refined HMD by normal distillation.
It is possible to produce Nylon 6 using ACN as the monomer. The ACN can, in turn, be manufactured using the same equipment used to produce HMD. The only required change in processing is to partially hydrogenate, rather than completely hydrogenate, the ADN to produce a mixture of ACN and HMD, together with some unreacted ADN. When this is done, there is a greater than 30 fold increase in the level of PRI in the crude hydrogenation product. This is because under partial hydrogenation conditions a much smaller portion of the PRI is hydrogenated to HMI. In order to manufacture polymer grade HMD and ACN, it is necessary to remove the PRI from both products. U.S. Pat. No. 5,961,788 describes a process for removing PRI from ACN using reactive distillation with caustic.
In order to develop a commercially acceptable process to co-produce polymer grade HMD and ACN, it is necessary to find a way to force almost all of the PRI to co-distill with the ACN, thereby producing HMD that has an acceptable level of PRI. There is a need, therefore, for a distillation process that allows the recovery of HMD that is substantially free of PRI, and which causes substantially all of the PRI to co-distill with the ACN.